_Zostera_ (Grass-wrack) is an entirely submerged, marine plant with creeping rhizome (with displacement of buds) and strap-shaped leaves. The flowering shoots are sympodia with displacement of the axes (Fig. 281). The inflorescence is a peculiar, flatly-compressed spike, on _one_ side of which the flowers are borne (Fig. 281). ~This inflorescence may be considered, no doubt correctly, to be derived from the symmetrical spike of _Potamogeton_ by strongly dorsiventral development, and by a strong suppression of the floral parts taking place simultaneously. Two rows of flowers are developed, but of these one is so pressed into the other that apparently only one is present.~ Each flower consists of only 1 stamen and 1 carpel situated at the same height (Fig. 281); the unilocular ovary encloses 1 pendulous ovule and bears a bifid style. As regards the perianth (?) one leaf may be present (_Z. nana_, Fig. 281 _D_). The pollen-grains are filamentous. Pollination takes place under water. ~_Posidonia_ and _Cymodocea_ are allied to these. About 70 species.~ [Illustration: FIG. 281.--_Zostera._ A Diagram of the branching of the floral shoots: _I_, _II_ ... are the successive shoot-generations, every other one being shaded; _g_{1}_ _g_{2}_ ... fore-leaves; _sp_{1} sp_{2}_ ... spathes for the successive spikes. Each shoot is united for some distance with the parent axis (indicated by the half-shaded internodes). Each shoot commences with a fore-leaf turning towards the parent axis, _g_; succeeding this is the spathe, _sp_; and then the inflorescence. The fore-leaf supports a new lateral shoot. _B_ Diagram of a shoot, _II_, which is borne laterally in the axil of the fore-leaf _g_{1}_, on the shoot _I_, _g_{2}_ its fore-leaf; _sp_{2}_ its spathe; _sti_ squamulæ intravaginales. _II_ Is the spadix with stamens and carpels; _b_ a perianth-leaf (or connective expansion, similar to those which occur in _Potamogeton_). _C_ The upper portion of a young spadix with development of flowers. _D_ Part of a spadix with 2 flowers; the parts which theoretically belong to one another are connected by a dotted line.] Order 3. =Aponogetonaceæ.= Aquatic plants with tuberous stem. They have a single, petaloid perianth (3–2–1–leaved), most frequently 6 stamens and 3(-6) carpels. Straight embryo.--About 15 species (Africa, Madagascar, Tropical Asia and Australia).--_Aponogeton distachyos_ and _A._ (_Ouvirandra_) _fenestralis_ are grown in conservatories; the latter has lattice-like, perforated leaves.